The present invention relates to a spindle apparatus and, more particularly, to a spindle apparatus using a magnetic bearing. In some spindle apparatus, in order to realize a super high speed rotation, or a long term non-maintenance or the like, a magnetic bearing is leviated floated without any contact with a rotary shaft (i.e., rotary body) by a magnetic force. The magnetic bearing is so constructed that an output of a positional shift sensor for detecting a position of the rotary shaft is fed back to control an excited current of an electromagnet and to control the floating position of the rotary shaft. In this feedback operation, if an imbalance (offset between an axial center and a gravitational center) is present in the rotary shaft, a magnetic force which is in synchronism with a rotary frequency (i.e., rpm) is generated in order to suppress a vibratory rotation of the rotary shaft which is generated due to the imbalance. Accordingly, when the rotary frequency is equal to a natural frequency (resonant frequency), the rotary shaft is resonated by the magnetic force of the magnetic bearing, as a result of which, in particular, in a high speed rotational region, the rotary shaft is deformed to cause a bending vibration.
FIG. 3 shows a state in which a rotary shaft 10 that is floatingly held by a magnetic bearing is subjected to the bending vibration as indicated by dotted lines with nodes at points A and B.
As shown in FIG. 3, the rotary shaft 10 is supported at both ends thereof by four electromagnets 12, 14, 16 and 18. The positional shift in a radial direction of the rotary shaft 10 is detected by positional shift sensors 20, 22, 24 and 26. In general, in the bending vibration, the nodes of the vibration are generated in the vicinity of both ends of the shaft, and also, the support positions of the magnetic bearing, i.e., the electromagnets are located in the vicinity of both ends of the shaft. Accordingly, as shown in FIG. 3, the nodes of the vibration (points A and B) are located in the vicinity of the electromagnets.
Usually, a circuitry or the like for phase compensation on the basis of a PID control (proportional-integral-derivative control) is incorporated into a control circuit for controlling the excited current of the electromagnets in the magnetic bearing. In order to suppress the resonant vibration, an electric damping is applied by using the magnetic force of the electromagnets and the current in the vicinity of the resonant frequency is interrupted by using a filter.
Also, in the prior art, a mechanical damper made of rubber material has been used at a portion C which is a middle portion in the vibration of the rotary shaft 10, thereby suppressing the generation of the bending vibration.
However, as shown in FIG. 3, if the points A and B that are the nodes of the vibration are located at the vicinity of mount positions of the respective electromagnets 12, 14, 16 and 18, the magnetic force of the electromagnets is not applied to the rotary shaft 10 as a force for suppressing the vibration. Accordingly, in the electric damping control using the above-described compensation circuit and the like, it is impossible to suppress the generation of the resonance. Also, in the case where the current in the vicinity of the resonance frequency is interrupted by using the filter, a rigidity of the magnetic bearing is degraded so that the rotary shaft 10 per se is likely to be vibrated by disturbance.
With the mechanical damper, it is possible to suppress the vibration of the middle portion C, but in this case, it is impossible to actively suppress the bending vibration unlike with the electric damping control. Also, the mechanical damper also suffers from a problem in durability.